Types of Mycorrhizal Symbioses

Mycorrhiza play a significant role on the stabilization of the plant communities in all terrestrial ecosystems.

There are six different yet well defined types of mycorrhizal symbioses in nature. The differences among these types of mycorrhizae, are based on the specific groups of fungal symbionts, patterns of root colonization, and the range of host plants that they are able to colonize.

Mycorrhizal symbioses are divided in:

a) Ericoid Mycorrhiza

This symbiosis is common in plants belonging to the Ericaceae, Epacridaceae and Empetraceae families in the order Ericales. There is controversy over the identity of the fungal partner in this symbiosis. The Ascomycete Hymenoscyphus ericae is the most studied fungus is this symbiosis. Other fungus involved in the ericoid mycorrhizal is Scytalidium vaccini and members of the genus Oidiodendron.

b) Monotropoid Mycorrhiza

This symbiosis is typical in achlorophyllous plants belonging to the ten genera from the family Monotropaceae. Examples of these plants are Monotropa hypopitys, M. uniflora, Pterospora andromeda, and Sarcodes sanguinea.

These plants make a strategical connection with the ectomycorrhizal mycelium from few fungi associated to the main host. Then, the achlorophyllous plants are able to take carbon (C) derived from the symbiosis between fungus-host. This is the only way for these plants to obtain C-compounds required for their physiological metabolism.

c) Arbutoid Mycorrhiza

This symbiosis is formed when a few of ectomycorrhiza-forming fungi are associated to the root system of Arbutus menziessi, Arctostaphylos uva-ursi, Pyrola rotundifolia and P. secunda, and plants of the genus Rhododendron.

d) Orchid Mycorrhiza

The members of the Orchidaceae are mycorhizal. The most common fungal genera able to form symbiosis with orchids are Rhizoctonia (R. solani, R. repens, and R. goodyera-repentis), Ceratorhiza, Thanatephorus, Ceratobasidium, Ypsilonidium, Sebacina and Tulasnella.

e) Ectomycorrhiza

This symbiosis is formed between fungi that belong to the following fungal divisions Basidiomycota, Ascomycota, and Zygomycota. There are more than 5000 species of ectomycorrhizal-forming fungi. They specifically colonize plants of the following botanical families: Betulaceae, Pinaceae, Fagaceae, and some members of the Fabaceae (Leguminoseae), Rosaceae, and Juglandaceae. The anatomical structure of this symbiosis is easily visible to the human eye (refer to Gallery of Pictures).

f) Arbuscular Mycorrhiza

This symbiosis is found in more than the 80% of all terrestrial plants. The fungi responsible for this symbiosis belong to the fungal division Glomeromycota, which comprise of aproximately 150 species of arbuscular mycorrhizal fungi (AMF). The anatomical structure of this symbiosis is only possible to distinguish it by using light microscopes. The typical structures that AMF form inside roots are: arbuscules, vesicles, hyphae, and some of them form also intraradical spores (refer to Gallery of Pictures).

Ectomycorrhiza and arbuscular mycorrhiza symbiosis are the most significant to agriculture/horticulture/forestry.